Your son would be having insect toxic dermatitis
There is swollen foot with erythematous lesion with centrally white hue.
There may be seconadary bacterial infection ,which cause oedema and pain.
Antibiotics like cephalexin
,anti inflammatory drugs and intihistaminics may be given.
Apply antibiotic cream like sisomycine.
Elevation of affected foot improve the disease.
You consult dermatologist
I hope you got my answer.
Dr. Ilyas Patel MD